1. Field of the Invention
The present invention relates generally to digital demodulators and more specifically to an improved technique for synchronizing a demodulator to the frequency and phase of a modulated carrier signal. This technique is particularly advantageous when the modulation format of the carrier signal is any one of a known plurality of different modulation formats. In particular this invention relates to a method and apparatus for demodulating a modulated carrier and for producing phase- and gainerror signals in the carrier recovery loop of a demodulator programmed for a particular phase and amplitude modulation format.
2. Description of the Prior Art
There are a number of different modulation schemes where the phase and/or amplitude of a carrier signal are varied to transmit digital information. Six of the most commonly used digital modulation formats are depicted by the phase and amplitude state diagrams shown in FIGS. 1A-1F. The phase shift keying (PSK) format is illustrated by the constellation patterns in FIGS. 1A and 1B. The quadrature amplitude modulation (QAM) format is illustrated by the constellation patterns in FIGS. 1D and 1F. 3- and 7-level quadrature partial response format signals (also referred to as 9 QPR and 49 QPR respectively) are illustrated by the constellation patterns in FIGS. 1C and 1E. (The phrase "constellation pattern" is used herein in its normal sense, i.e. the two-dimensional diagram of the in-phase and quadrature phase signal components.) The amplitude-phase keying (APK) formats considered here include multilevel QAM, arbitrarily mapped APK, symmetrical APK, and unsymmetrical APK.
The modulation formats illustrated in FIGS. 1A-1F can be divided into two general classes. The first class is characterized by PSK modulation, examples of which are illustrated by the constellation patterns in FIGS. 1A and 1B. In a phase-modulated signal the carrier amplitude is held constant as its phase is rapidly shifted by different multiples of a phase interval. There are normally an even number of possible phase states in this type of modulation format, typically 2, 4, 8, 16, etc. In the 8-phase PSK format, data is modulated by the phase of the carrier abruptly changing from one phase to another by some multiple of 45.degree. phase shift.
In the second class of modulation formats, not only is the phase changed between data points, but the amplitude of the modulated signal can also change. The remaining four constellation patterns shown in FIGS. 1C-1F are examples of the second class of modulated signal formats: 3-level partial response, 16-QAM, 7-level partial response, and 64-QAM modulation formats. Commerical telephone modems typically use both classes of modulation formats.
Most state-of-the-art communication receivers are designed to demodulate a particular class of modulated signals. This assures that a receiver is not only capable of demodulating the particular modulation format used by the transmitter, but also that it is efficient for the type of modulation in use. It does, however, limit the receiver to working with a particular class of modulated signals. Even special purpose receivers are limited to demodulating a carrier modulated with a modulation format from only one of the two classes of modulation types. This is because no single demodulating technique exists which is capable of handling all types of modulation formats (except for switchable receivers which in reality is not a single demodulator at all since it has multiple demodulators). A particular carrier recovery system (demodulator) that uses a ROM (read only memory) table look-up method of sequentially deciding which data points of a signal constellation pattern are actually transmitted is disclosed in CARRIER RECOVERY SYSTEMS FOR ARBITRARILY MAPPED APK SIGNALS by Yoshio Matsuo and Junji Namiki, IEEE Transactions on Communications, Vol. COM-30, No. 10, October 1982, pages 2385-2390. The demodulator is optimized for fast acquisition and has a one bit error output--the error being an indication that the data point is not within the center of the decision zone of the demodulator. Phase and amplitude errors needed to lock the receiver onto the carrier are measured using dedicated circuitry. The author does not address more sophisticated error output implementations. A carrier recovery system that uses a different approach to track PSK signals is disclosed in A GENERALIZED "POLARITY-TYPE" COSTAS LOOP FOR TRACKING MPSK SIGNALS by Holly C. Osborne, IEEE Transactions on Communications, Vol. COM-30, No. 10, October 1982, pages 2289-2296. This reference describes a system of x and y quantization usable in PSK systems. It does not address the possibility of new data states nor does it address how to resolve data states from these new and ambiguous states. As an example, a 16-QAM tracker can lock onto 8-PSK signals very well. However the eight extra phase and amplitude states do reduce the trackers functioning in a low signal-to-noise environment, as pointed out in the paper, and the eight new data values created are ambiguous.
Typically a plurality of different receivers (or family of receivers), each designed to accommodate a different modulation type or signal constellation pattern, selectively receives and demodulates a carrier signal having any one of a number of different modulation formats. This approach, as discussed therein, has the distinct disadvantage of using a large amount of hardware.
The subject invention overcomes these drawbacks by employing a demodulation technique and receiver capable of recovering data from any modulated carrier signal if the modulation format is apriori known. By "programming" the receiver with the constellation pattern for a particular modulation format, the receiver is capable of demodulating a carrier signal having that particular modulation format.
An object of this invention is the provision of a universal carrier recovery circuit for the demodulator of a digital data communication system.